1. Field of the Invention
The invention relates generally drill bits used to drill wellbores through earth formations. More specifically, the invention relates to a bit structure having reduced wear and method for applying hardfacing so as to reduce erosion of the drill bit during drilling operations.
2. Background Art
Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as xe2x80x9cfixed cutterxe2x80x9d or xe2x80x9cdragxe2x80x9d bits, which usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements may be formed from any one or combination of hard or superhard materials, including, for example, natural or synthetic diamond, boron nitride, and tungsten carbide.
Drill bits of the second category are typically referred to as xe2x80x9croller conexe2x80x9d bits, which usually include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits are typically referred to as xe2x80x9cmilled toothxe2x80x9d bits. Other roller cone bits include xe2x80x9cinsertxe2x80x9d cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
Application of hardfacing to the base material from which the cones and drill bit are formed is known in the art. The hardfacing can be applied in the form of special erosion protection inserts used in addition to the cutting elements. See for example, U.S. Pat. No. 3,952,815 issued to Dysart. Another method known in the art that uses hardfacing to protect insert bit roller cones is described in U.S. Pat. No 5,291,807 issued to Vanderford. The method in the Vandeford ""807 patent includes marking the face of a roller cone by masking or etching, applying hardfacing material, such as tungsten carbide, in the form of a powder, and heating the cone to bond the hardfacing powder to the cone. This technology has shown problems of concentration erosion of cone steel between carbide insert and hardfacing patch. U.S. Pat. Nos. 3,461,983 and 3,513,728 issued to Hudson include disclosure related to drilling holes (sockets) in the cone prior to application of the hardfacing, plugging the holes, and then applying the hardfacing material using a flame application process. After applying the hardfacing material with the flame process, the plugs are removed and the inserts are pressed into the previously drilled sockets. One issue with this technology is distortion from quench operation or damage to cone due to thermal mismatch between plug and cone. Note, this technology requires the plug to remain in the insert hole through processing.
Moreover, U.S. Pat. No. 5,348,770 issued to Sievers discloses a method for applying hardfacing to a cone which uses a high velocity oxygen fuel (HVOF) spray process after the cone is formed. Forming the cone includes drilling the sockets for the inserts. Those skilled in the art know that HVOF coatings applied to a finished or formed cone have poor impact and bonding due to temperature limitations, that is, temperature is restricted by either the steel temper or required carburized case. Both effectively limit cone heating to below 500xc2x0 F. U.S. Pat. No. 4,396,077 issued to Radtke discloses a method for applying hardfacing to a fixed cutter bit. The method includes generating an electric arc and spraying arc-heated hardfacing material onto a substantially completely assembled bit structure.
With respect to milled tooth bits, regardless of how the hardfacing is applied, the hardfacing material resists wear as the gage row teeth cut a gage of an earth formation. As the gage row milled teeth wear, along with the hardfacing material, the gage of the borehole will be reduced depending on the amount of wear of the gage row milled teeth. As the gage row milled teeth continue to wear, the cutting capability of the cone is reduced.
What is needed, therefore, is a structure and method for enhancing the durability of gage row milled teeth without sacrificing reliability of the cutting structure.
In one aspect, the present invention relates to a drill bit including a bit body having at least one roller cone rotatably coupled to the bit body with a plurality of milled teeth formed on the at least one roller cone, at least one milled tooth arranged so as to form a gage row milled tooth, the gage row milled tooth includes hardfacing thereon, and a cutting element insert mounted in the gage row milled tooth.
In another aspect, the present invention relates to a method of forming a drill bit structure, the method including machining at least one hole in a preselected location on a gage surface of at least one milled tooth, positioning a plug in the at least one hole, applying a hardfacing material to the at least one milled tooth, removing the plug from the at least one hole, and positioning a drilling insert in the at least one hole. Critical to this process is maintenance of gage dimension or outside diameter of bit. Loss of gage dimension due to wear of cutting structure results in decreased bit performance as well as increase operating costs due to hole reaming or cleaning.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.